The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition (SI) engines introduce an air/fuel mixture into each cylinder which is compressed in a compression stroke and ignited by a spark plug. Known compression ignition engines inject pressurized fuel into a cylinder when a piston is near top dead center (TDC) of a compression stroke, and the injected fuel ignites upon injection. Combustion for both gasoline engines and diesel engines involves premixed or diffusion flames controlled by fluid mechanics.
SI engines can operate in a variety of different combustion modes, including a homogeneous SI combustion mode and a stratified-charge SI combustion mode. SI engines can be configured to operate in a homogeneous-charge compression-ignition (HCCI) combustion mode, also referred to as controlled auto-ignition combustion, under predetermined speed/load operating conditions. HCCI combustion is a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in an HCCI combustion mode has a cylinder charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. HCCI combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute air/fuel mixture, i.e., lean of a stoichiometric air/fuel point, with relatively low peak combustion temperatures, resulting in low NOx emissions. The homogeneous air/fuel mixture minimizes occurrences of rich zones that form smoke and particulate emissions.
Engine airflow is controlled using an air intake system including a throttle valve and intake valves and exhaust valves. On engine systems so equipped, opening and closing of the intake valves and exhaust valves can be adjusted using a variable valve actuation system that includes variable cam phasing and a selectable multi-step valve lift, e.g., multiple-step cam lobes which provide two or more valve lift positions. A throttle position change is continuous, whereas changes in valve open positions of intake and exhaust valves controlled using multi-step valve lift mechanisms is discrete.
When an engine operates in a HCCI combustion mode, the engine operates at a lean or stoichiometric air/fuel ratio operation with the throttle valve wide open to minimize engine pumping losses. When the engine operates in the SI combustion mode, the engine operates in stoichiometric air/fuel ratio, with the throttle valve controlled over a range of positions from 0% to 100% of the wide-open position to control intake airflow to achieve the stoichiometric air/fuel ratio.
In an engine configured to operate in both SI and HCCI combustion modes, transitioning between combustion modes can be complex. Known engine control systems coordinate activations of multiple devices in order to provide a desired air/fuel ratio during operation in the different modes. During a transition between a HCCI combustion mode and an SI combustion mode, valve lift switching occurs nearly instantaneously, with a corresponding effect upon intake airflow, whereas adjustments to throttle valve opening and cam phasing have slower dynamics with slower corresponding effects upon intake manifold pressure and airflow. It is known that incomplete combustion and misfire leading to torque disturbances may occur during a transition due to an incomplete understanding of intake airflow dynamics and a corresponding inability to effectively control fueling and air/fuel ratio.